FIELD OF THE INVENTION
The present invention relates to a means and method for hemming sheet metal or like material by converting perpendicular motion into horizontal bending forces and more particularly to rotary bending tools.
Manufacturers today form many parts by a stamping operation in a press brake, a punch press or by automation/special operation machine equipment. The term "press" is used in this specification to collectively refer to all such force engines. Conventionally, the press is provided with a set of dies appropriately mounted on the ram and bed of the press and the ram reciprocated with respect to the bed as sheet metal or similar material is fed into the press between the dies. Using the pressure provided by the press, the die faces act against the sheet forcing it to conform to the contours of the die faces.
Often, one or more portions of the sheet is to be bent back against itself or wrapped over another layer of sheet and bent to hold the two sheets fast to one another. Prior art accomplished hemming operations of this type through two methods.
I. Two Operations/Separate Die Sets. The first method involved sequential use of two die sets. The first operation on the sheet utilized a V-shaped die set, as shown in FIG. 1, which upset the sheet by the pressure of the press against the die set which in turn acted against the interposed sheet to bend the sheet to greater than ninety degrees. As shown in FIG. 2, in the second operation, a die set having flat contact faces forced the bent portion, or skirt, formed in the first operation, flat against the parent sheet. In the first operation, it was necessary to form the upset skirt at an acute angle relative to the parent sheet, and the smaller the angle the better, to assure that the flat-faced die used in the second operation continued to bend the skirt toward the parent sheet rather than away from it. If another sheet was to be hemmed between the parent sheet and its skirt, due to the acute angle of the bent skirt, the additional sheet had to be slid in from the side, rather than dropped from the top onto the parent sheet.
Using this method of hemming, the first die set presented the greatest obstacle to efficiency in press operation. By virtue of the way the die set acted against the interposed material the sheet could not be laid perpendicular to the direction of ram travel. Instead, the sheet had to be held parallel to one forming edge of the V-shaped die. Otherwise, as the die pair acted against the sheet, the sheet would bend at the edges of the die faces as well as at the desired location between the die faces. Consequently, to accommodate proper support of the sheet in the press, the press bed had to be positioned other than perpendicular to the direction of ram travel, or, alternatively, the operator had to position and hold the sheet by hand during press operation forming the skirt. This method of hemming made production set-up and operator control difficult, and made machine operation inefficient. If the operator was required to position the sheet by hand, this led to operator fatigue and improperly bent pieces. If hemming was to be accomplished with the minimum number of operations, the flat faces of the second die set required that the first die set produce a skirt bent at an acute angle with respect to the parent sheet to assure that the skirt would bend toward the parent material when acted on by the second die set. Thus, the manufacturing operation faced a series of trade-offs involving the number of bending operations to complete a hem, the method of introducing additional sheets against skirted sheets, and positioning of sheets for initial skirt upset.
II. Single Station Operation/Combination Die Sets. The second method of prior art press bending combined the two die sets described above into a single combination die set commonly known as a "three-high" hemming die set. As shown in FIG. 3, in the three-high hemming die set an upper V-shaped die set upset the sheet material to form an acute angle between the skirt and the parent material while the lower flat-faced die set flattened the upset skirt against the parent sheet after initial overbending by the first die set. Sheet material is shown in both bending dies in FIG. 3 for illustration only, both dies not normally being used simultaneously.
The three high die set did not overcome the shortcomings of the sequential die operations described in the first method above. The sheet material still had to be positioned parallel to one working surface of the V-shaped die to avoid undesirable edge bends. For certain applications, the three high die demonstrated another shortcoming. Sheet insertion depth was limited by the central connection of the upper die to its companion lower die. Consequently, following working of the sheet, the sheet must be pulled backwards from the direction of insertion. This increased the difficulty of usage with a multi-station press since it blocked passage to insert the piece from one side and remove it from the other.
To increase productivity and efficiency, fabricators have recognized the value of reducing the number of hemming operations. Ideally, a hemming machine or operation should upset a skirt completely around the parent sheet in a single operation. Additional sheets would then be loaded directly against the parent sheet inside the upset skirt and the skirt hemmed against the sheets in a single bending operation. To facilitate insertion of additional sheets, the angle of the upset skirt with respect to the parent sheet would be obtuse to enable insertion of additional sheets from above.
Conventional hemming methods will not allow for this, requiring either that additional sheets be bowed for insertion from above the parent sheet, or that one end of the parent sheet not be upset so that additional sheets could slide in from the side, or that more than two bending operations be performed to gradually collapse the skirt about the additional sheets to hem them together. Clearly, there is need for a hemming tool which would minimize the number of operations needed to hem yet which permits easy introduction of sheets to be hemmed against one another.
An additional problem with hemming metal sheet is spring back, the tendency of the skirt to not remain flat against the parent sheet but to remain at some acute angle with respect to the parent sheet. To overcome spring back, the final operation on the skirt must "coin" the skirt, that is, press the skirt, notably at its bend, tightly against the parent sheet and work the metal sufficiently to remove spring back. Often, conventional hemming methods required an additional coining operation to produce a satisfactorily hemmed piece.